1. Field of the Invention
The present invention relates to a transistor that is applicable to electronic devices, and more particularly to a transistor of nanometer size that operates at room temperatures.
2. Description of the Prior Art
Fullerene and carbon nanotube discovered in recent years are in the limelight as a new carbon substance different from graphite, amorphous carbon, and diamond that had been known until then. This is because the fullerene and carbon nanotube exhibit specific electronic physical properties different from those of existing carbon substances.
For example, fullerene typified by C60 and C70 contains many carbon atoms placed in spherical cage form to constitute one molecule, and dissolves in organic solvents such as benzene. There are many kinds of fullerene including C60 and C70 that exhibit the nature of superconductors and semiconductors. Also, the fullerene has a high light-sensing effect and is expected to be used as an electrophotographic photosensitive material. Furthermore, fullerene can exhibit effective physical properties as a functional material when a different kind of element is confined inside it, or many kinds of chemical functional groups are added to the outside thereof.
Carbon nanotube is a new material containing only carbon, like fullerene. Functions such as an electron emission source, a semiconductor material, a hydrogen storage material have been discovered. Particularly, since it can function as a semiconductor or conductor according to a slight change in the chirality of atom array, it is expected to be used as a switching device of nanometer size in various fields of the electronics industry.
On the other hand, for silicon devices that are dominant electronic devices, with the development of advanced microfabrication techniques, the gate electrode width of a field effect transistor (FET) is miniaturized to about 0.1 xcexcm, and owing to an increased level of integration, a memory having an operation speed of about 1 Gbit is prototyped. The most advantageous point of a silicon device is that, in the case where a silicon oxide is used as an insulator, the interface level between the silicon and the silicon oxide is remarkably low, and an oxidation MOS (metal oxide semiconductor) transistor can be easily formed. By using the small-sized MOS transistor having low power consumption in logical circuits, a high level of integration of devices has become possible. A highly pure silicon material is produced by the halogen process, while a semiconductor wafer 30 cm or more in diameter is fabricated by the Czochralski crystal growth method, so that the productivity of devices is extremely high.
However, silicon is low in carrier mobility and has a limited switching speed. The drawbacks have been solved by the GaAs field-effect transistor (GaAs-FET) and GeSi bipolar transistor. The carrier mobility of GaAs is higher than that of silicon and the GaAs-FET has much higher operation speed than Si transistors. Also, a GeSi bipolar transistor, although comparable to the GaAs-FTE in operation speed, is being frequently used in portable terminals and the like because of its cheap device unit cost.
Furthermore, to achieve a switching speed of several tens of GHz, HEMT (high electron mobility transistor) produced by two-dimensional electronic gases in which electrons and holes are two-dimensionally confined was devised. At present, these devices are electronic devices indispensable to high-frequency communications of several GHz or more, including mobile communications.
At present, electronic devices expected for higher operation speeds are ones having a low-dimension structure such as quantum thin lines and quantum dots. By confining electrons and holes one-dimensionally (line) or 0-dimensionally (dot), it is conceivable that ultrafast operation is achieved. The low-dimension structure of semiconductor devices not only breaks the limit of device size but also is expected as an important technology for achieving ultrafast operation of switching devices.
Particularly, since carbon nanotubes have a diameter as small as several nm, their electrical conduction mechanism is equal to one dimension and they are is in the limelight as low-dimension conductive substance. Since some single-wall carbon nanotubes exhibit semiconductor characteristics, they have a latent capability to form transistors of nanometer size from the carbon nanotubes. At present, the commutation characteristic of the carbon nanotubes at ordinary temperatures is confirmed, and one-dimensional quantum condition (Luttinger liquid condition) at room temperatures is also experimentally suggested. Therefore, by applying a ballistic conduction mechanism at ordinary temperatures, it is conceivable that switching devices of the carbon nanotubes having an operation speed of several THz can be realized.
Also, the processing process of silicon devices have many problems for further microminiaturization and nears technological limitations. Particularly, in exposure technologies, for technologies of a line width of 0.1 xcexcm or less, which is an optical limit, although the F2 laser exposure method, the electronic beam exposure method, and the like are proposed, they have many problems in oxidation film formation and others. At present, although various technologies are devised to realize devices that operate at sizes of 0.1 xcexcm or less and the devices are actually developed, there are many problems as fabrication technologies.
Therefore, if a technology of fabricating electronic devices by carbon nanotubes is offered, it is expected that devices which are capable of high speed operation and are alternative to the processing process of silicon devices coming near the limit can be proposed. In xe2x80x9cThe Journal of Physical Chemistry B. Vol. 103, No. 36, 1999, pp. 7551-7556xe2x80x9d, a method of producing carbon nanotubes and the electrically linear characteristics (conductivity characteristics) of obtained carbon nanotubes are described.
However, carbon nanotubes produced by arc discharge, laser ablation, and the like are almost constant in thickness but different in length, from several tens of nm to several mm, and have difficulty in controlling the length thereof. A technology of obtaining carbon nanotubes of a size necessary to form devices is presently not available. With prior arts, carbon nanotubes of a size incidentally obtained are only used, and although carbon nanotubes can be experimentally used, it has been difficult to industrially use them as electronic device materials.
Connections of carbon nanotubes with metallic electrodes have counted against high-speed operation because of high contact resistance.
Accordingly, the present invention has been made in view of the above circumstances and provides a transistor of nanometer size that is capable of high-speed operation and operates at room temperatures by using carbon nanotubes for semiconductor devices.
It is known that ultrasonically processing single-wall carbon nanotubes causes minute rings, that is, carbon nanotube rings referred to in the present invention to be formed. The present invention is characterized in that a transistor as an electronic device is formed using the carbon nanotube rings.
The present invention provides a transistor using a carbon nanotube ring having semiconductor characteristics as a semiconductor material.
The present invention also provides a transistor using a carbon nanotube ring having conductivity or semiconductor characteristics as an electrode material.
The present invention also provides carbon nanotube rings having p-type semiconductor characteristics.
The present invention also provides a semiconductor device in which carbon nanotube rings having the p-type semiconductor characteristics are placed on an n-type semiconductor substrate thereof.
The present invention succeeds in forming a transistor of stable quality by using carbon nanotube rings in which carbon nanotubes are formed in ring shape. This is because carbon nanotube rings made of carbon nanotubes are obtained with a small range of variations of sizes (outside ring diameter) thereof, and the obtained carbon nanotube rings can be classified by size, so that carbon nanotube rings of stable size can be used as a semiconductor material or an electrode material.